The present invention relates to a photomultiplier tube designed to detect through multiplication weak light radiated onto a faceplate.
Photomultiplier tubes are produced at various sizes to suit different uses. Conventional photomultiplier tubes are generally configured of a hermetically sealed vessel constructed entirely of glass. This type of photomultiplier tube has a substantially cylindrical glass side tube with a planar and substantially circular stem fitted on the bottom open end. A gas burner is applied to the area in which the periphery of the stem contacts the inner surface of the side tube, and the two are fused together by heat to form an airtight seal. A planar and substantially circular transparent faceplate is fused in the same manner to the top opening of the side tube, forming an airtight seal. A glass exhaust tube is provided in the stem, forming a fluid connection with the inside of the hermetically sealed vessel. After evacuating the vessel through the exhaust tube, an alkaline metal vapor is introduced into the vessel to form a photocathode on the inner surface of the faceplate and also to activate the secondary electron emitting surface of the electron multiplying section disposed inside the vessel.
As described above, the side tube and stem are fused together with heat from a gas burner in order to construct a photomultiplier tube having an airtight glass vessel. Further, after introducing the alkaline metal vapor, the exhaust tube is pinched off and sealed with the gas burner. Accordingly, it is necessary to prevent heat from damaging the electron multiplying section. Therefore, it has been necessary to maintain a distance of about 20-30 mm from the airtight fused junctions to the bottom of the electron multiplying section. Unfortunately, this inhibits downsizing of the photomultiplier tube.
While it depends on the application, the advantages of downsizing a photomultiplier tube are generally great. One example is a sanitation monitor that is used for detecting bacteria in restaurants and the like. The sanitation monitor employs a photomultiplier tube to detect light that is emitted from reactions between drugs and bacteria. Since this type of monitoring device should be portable and easy to use, the photomultiplier tubes used in the monitor must be small. A small enough photomultiplier tube can be mounted directly on a circuit board and treated in the same way as a resistor or capacitor, thereby making the tubes more convenient for device construction.
In light of these demands, small photomultiplier tubes having metal side tubes have been developed for practical uses in recent years. An example of this type of photomultiplier tube described in Japanese Unexamined Patent Application Publication Nos. HEI-5-290793 and HEI-9-306416 has a metal side tube having a polygonal cross-section with a flange portion protruding laterally from the bottom of the tube. Similarly, the metal stem plate is provided with a flange portion that protrudes laterally. The flange portions of the side tube and stem plate overlap and are fused through resistance welding to form a hermetically sealed vessel. Resistance welding is performed in such a way that current is applied to the joining parts, which are heated through the heat generated from resistance. When the parts reach an appropriate temperature, pressure is applied to weld them together. Hence, it is possible to avoid the thermal affects that resistance welding has on the electron multiplying section. As a result, the distance between the stem plate and electron multiplying section can be reduced, thereby achieving a smaller photomultiplier tube that is shorter in the axial direction.
With a photomultiplier tube of this construction, however, the flange portions required for resistance welding interfere with the use of the tube. For example, when photomultiplier tubes are used in gamma cameras and the like, in particular, it is necessary to arrange a plurality of photomultiplier tubes closely together and form a large area for receiving light. With this configuration, the flange portions contact other flange portions, forming dead spaces. Dead spaces are a hindrance to achieving a high-performance detecting device.
The present invention has been made to solve the above-described problems, and accordingly it is an object of the present invention to provide a photomultiplier tube and a method for manufacturing a smaller multiplier tube.
The above object will be attained by a photomultiplier tube having a faceplate, a photocathode for emitting electrons in response to light incident on the faceplate, an electron multiplying section, disposed inside an airtight vessel, for multiplying the electrons emitted from the photocathode, and an anode for outputting an output signal based on the electrons multiplied by the electron multiplying section, characterized in that the airtight vessel is formed by:
a stem plate for fixedly supporting the electron multiplying section and the anode with stem pins;
a side tube with the stem plate fixed on one open end, the side tube being formed of metal and enclosing the electron multiplying section and the anode; and
the faceplate fixed on the other open end of the side tube, and
that an inner surface of a lower portion of the side tube is in contact with an outer edge of the stem plate, at least a portion of the stem plate contacting the inner surface of the lower portion of the side tube is formed of metal, the portion thereof including the outer edge of the stem plate, and the side tube is welded to the stem plate.
In the photomultiplier tube as defined above, the side tube is fixedly secured by welding to the stem plate while the inner surface of the lower portion of the side tube is maintained to be in contact with the outer edge of the stem plate, with the result that there is no projection like a flange at the lower portion of the photomultiplier tube. Therefore, though it is difficult to perform resistance welding, the outside dimensions of the photomultiplier tube can be decreased, and the side tubes can densely abut to one another even if the photomultiplier tubes are arranged when applied. Hence, high-density arrangement of photomultiplier tubes are realized by assembling metallic stem plate and the side tube by, for example, laser welding.
It is preferable that the side tube have a free end at the lower portion thereof, allowing the stem plate to slide along the inner surface of the side tube. With this configuration, the stem plate can be inserted through the open end of the side tube, such that the edge surface of the stem plate contacts and slides inwardly on the inner surface of the lower portion of the side tube for adjusting and determining the position of the stem plate. As a result, the distance between the electron multiplying section, which is fixed to the stem plate, and the photocathode can easily be adjusted before the welding process.
It is further preferable that the metal side tube is fusion welded to the stem plate. In contrast to resistance welding, a fusion welding method does not require that pressure be added to the portions of the side tube and stem plate that are being joined. Accordingly, residual stress that can lead to cracking during operations is not generated at this junction, thereby greatly improving durability of the apparatus.
It is preferable that the fusion welding be laser welding or electron beam welding. The methods of laser welding and electron beam welding generate little heat at the junction. Therefore, the glass tablets fixing the stem pins to the stem plate are not apt to crack from the effects of the heat when the stem pins are near the side tube. Accordingly, it is possible to move these stem pins closer to the side tube and expand the electron multiplying section laterally, creating a greater electron receiving surface area in the electron multiplying section.
Entirety of the stem plate can be formed of metal. Otherwise, the stem plate may include a metal stem support member, and a glass stem plate wherein the metal stem support member is in contact with the inner surface of the lower portion of the side tube.
A tapered surface can be formed in the inner surface of the lower portion of the side tube and a tapered edged surface can be formed in the stem plate to conform to the tapered surface of the side tube, so that the tapered surface of the side tube and the tapered edged surface of the stem plate are in contact with each other.
In addition, a bottom surface of the stem plate and a bottom surface of the side tube may be flush with each other.